Meredith Baxter and Michael Gross endeared themselves to millions of viewers during the seven seasons they spent starring together on the hit sitcom “Family Ties.” Now they rekindle their dramatic chemistry in a production of “Love Letters” by A.R. Gurney, coming to CAC for just one performance.  “Love Letters” centers on the separate but entwined lives of two pen pals, who through 50 years of letter-writing share with one another their hopes, ambitions, disappointments, victories, defeats, and dreams.